This invention relates to a steam generating apparatus for supplying nonradioactive, clean steam as sealing steam to shaft sealing sections of a steam turbine of a nuclear power plant or the like.
Heretofore, in a steam generating apparatus for supplying nonradioactive steam as sealing steam to shaft sealing sections of a steam turbine of a nuclear power plant, it has been a common practice, as described in U.S. Pat. No. 3,604,206, to introduce condensate from a condensate storage tank into a shell containing feed water below the liquid level which is kept constant at all times. Steam for heating the condensate is supplied to heating pipes submerged in the feed water in the shell and the feed water is evaporated by the heat exchange taking place between the feed water and steam through the pipes, and the generated steam is supplied as sealing steam to the shaft sealing sections of the steam turbine.
In this type of steam generating apparatus, some difficulty would be encountered when the steam turbine is started. At turbine start-up, the demand for a supply of sealing steam to the shaft sealing portions is increased transiently and suddenly because the supply of sealing steam to the shaft sealing sections has been interrupted. This increase in the demand for a supply of sealing steam would cause an increase in the demand for a supply of condensate to the interior of the shell as feed water in proportion to the increase in the demand for a supply of sealing steam. Since the condensate is supplied to the interior of the shell from below the liquid level, the water in the shell would have its temperature lowered by the supply of condensate of low temperature which is temporarily increased in quantity, and evaporation of the water in the tank would not take place vigorously. Thus it would become impossible to obtain steam having a pressure and temperature necessary to the sealing steam. Stated differently, the steam generating apparatus of the type described would have the disadvantage of being poor in responding to a change in the quantity of the sealing steam required for the shaft sealing sections of the steam turbine, with a result that the shaft sealing would become ineffective temporarily.